linux-kernel - Re: [PATCH 0/3] Virtual huge zero page

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Date:	Sat, 29 Sep 2012 16:37:37 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH 0/3] Virtual huge zero page

On Sat, Sep 29, 2012 at 07:30:06AM -0700, Andi Kleen wrote:
> On Sat, Sep 29, 2012 at 03:48:11PM +0200, Andrea Arcangeli wrote:
> > On Sat, Sep 29, 2012 at 02:37:18AM +0300, Kirill A. Shutemov wrote:
> > > Cons:
> > >  - increases TLB pressure;
> > 
> > I generally don't like using 4k tlb entries ever. This only has the
> 
> From theory I would also prefer the 2MB huge page.
> 
> But some numbers comparing between the two alternatives are definitely
> interesting.  Numbers are often better than theory.

Sure good idea, just all standard benchmarks likely aren't using zero
pages so I suggest a basic micro benchmark:

   some loop of() {
      memcmp(uninitalized_pointer, (char *)uninitialized_pointer+4G, 4G)
      barrier();
   }

> 
> > There would be a small cache benefit here... but even then some first
> > level caches are virtually indexed IIRC (always physically tagged to
> 
> Modern x86 doesn't have virtually indexed caches.

With the above memcmp, I'm quite sure the previous patch will beat the
new one by a wide margin, especially on modern x86 with more 2M TLB
entries and >= 8MB L2 caches.

But I agree we need to verify it before taking a decision, and that
the numbers are better than theory, or to rephrase it "let's check the
theory is right" :)
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